12/10/2025
During MEPC 83 in April 2025, the IMO adopted Resolution MEPC.397(83), which introduces updates to the NOx Technical Code 2008. These revisions focus on engine re-certification requirements and formalize the use of Multiple Engine Operational Profiles (MEOP), in addition to providing clearer guidance on the existing test cycle framework.
Applicability: shipowners, technical managers, operators, masters, shipyards, and engine manufacturers.
The revisions revise the existing certification pathway for marine diesel engines and confirm that several operational profiles may be applied for the same engine. The primary implementation milestone for new engines is set for 1 January 2028.
Scope of Application
The entry into force depends on engine category:
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New individual or parent engines certified for the first time must comply from 1 January 2028.
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New member engines joining an established engine group/family certified before 1 January 2028 must demonstrate conformity with the updated criteria by 1 January 2030.
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Existing certified engines must comply only if a major modification occurs on or after 1 January 2028.
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For identical replacement installations after 1 January 2028, the NOx Technical Code version applied is that in force when the EIAPP is issued, unless the replacement involves MEOP.
Additional clarifications regarding test cycle application are included but do not carry their own explicit enforcement dates.
Control Requirements for Multiple Engine Operational Profiles
Alternative operational profiles have been previously accepted in NOx certification—for example, engines approved simultaneously for Tier II and Tier III, or non-propulsion engines functioning under more than one duty cycle.
The new amendments formally authorize more than one operational profile for a single engine type so that differing load environments can be reflected. This is relevant for engines operating under variable conditions, such as auxiliary units serving alternately in sea-going mode or cargo loading mode, enabling fuel-use optimisation.
Where several profiles are applied, the final NOx figure is calculated using a composite approach: the highest NOx value at each mode point across every applicable profile determines the certification result.
These provisions are now incorporated into a newly added Chapter 8 of the Code.
Requirements for Rational Emission Control
The prohibition of irrational emission control has existed since the adoption of the 2008 MARPOL Annex VI update but had not been directly operationalized in the NOx Technical Code.
With the latest revisions, applicants must now verify that the engine’s emission control system demonstrates a consistent and predictable progression of NOx values between declared mode points across the operational load–speed spectrum. Exceptions apply only when auxiliary devices intervene for safety or operability.
Additionally, limits are introduced for how far emissions may deviate from the intermodal line. It is the applicant’s responsibility to define permissible deviations relative to the intended duty profile of the engine.
Clarification on Emission Test Cycles
The amendments refine how engine categories are labelled rather than altering the test cycles themselves. The former terminology of “main” vs. “auxiliary” is replaced by “propulsion” vs. “non-propulsion”, creating an unambiguous two-path classification. A process diagram has been added as Appendix IX to aid correct test selection.
Actions for Ship Owners and Operators
Stakeholders are advised to review these amendments promptly and undertake necessary compliance planning, particularly regarding certification documentation, operational test cycles, and MEOP application to ensure conformity before statutory enforcement dates.